Synthesis of PDMS-μ-PCL Miktoarm Star Copolymers by Combinations (Є) of Styrenics-Assisted Atom Transfer Radical Coupling and Ring-Opening Polymerization and Study of the Self-Assembled Nanostructures

Due to their diverse and unique physical properties, miktoarm star copolymers (μ-SCPs) have garnered significant attention. In our study, we employed α-monobomoisobutyryl-terminated polydimethylsiloxane (PDMS-Br) to carry out styrenics-assisted atom transfer radical coupling (SA ATRC) in the presence of 4-vinylbenzyl alcohol (VBA) at 0 °C. By achieving high coupling efficiency (χc = 0.95), we obtained mid-chain functionalized PDMS-VBAm-PDMS polymers with benzylic alcohols. Interestingly, matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis revealed the insertion of only two VBA coupling agents (m = 2). Subsequently, the PDMS-VBA2-PDMS products underwent mid-chain extensions using ε-caprolactone (ε-CL) through ring-opening polymerization (ROP) with an efficient organo-catalyst at 40 °C, resulting in the synthesis of novel (PDMS)2-μ-(PCL)2 μ-SCPs. Eventually, novel (PDMS)2-μ-(PCL)2 μ-SCPs were obtained. The obtained PDMS-μ-PCL μ-SCPs were further subjected to examination of their solid-state self-assembly through small-angle X-ray scattering (SAXS) experiments. Notably, various nanostructures, including lamellae and hexagonally packed cylinders, were observed with a periodic size of approximately 15 nm. As a result, we successfully developed a simple and effective reaction combination (Є) strategy (i.e., SA ATRC-Є-ROP) for the synthesis of well-defined PDMS-μ-PCL μ-SCPs. This approach may open up new possibilities for fabricating nanostructures from siloxane-based materials.


Introduction
Star copolymers (SCPs) are non-linear macromolecules with more than two linear polymer chains (arms) with a central core [1,2]. Among SCPs, homogeneous arms (e.g., homopolymer, random, or block copolymer segments) and heterogeneous arms (i.e., miktoarm SCPs (µ-SCPs)) are the two major classes of SCPs [3,4]. The former consists of arms with identical chemical structures, chain ends, and comparable compositions and molecular weights (MWs). The latter encompasses dissimilar arms with varying MWs, compositions, of monomers, timing of crosslinker addition, MW of arms, ratio of MMs/initiator, ratio of MIs/crosslinker, functionality, and composition of the polymeric arms [15,17,18].
Herein, we develop a simple and effective SA ATRC-Є-ROP strategy for synthesizing well-defined PDMS-µ-PCL (PDMS: polydimethylsiloxane) µ-SCPs. As shown in Scheme 1, the chain-end modification and SA ATRC of the PDMS-based precursors were conducted and examined by gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS). We employed a styrenic monomer, VBA, as a coupling agent to synthesize PDMS-VBA m -PDMS MI. An organo-catalyst catalyzed the ROP mid-chain extension of PDMS-VBA m -PDMS MI with ε-CL to afford well-defined µ-SCPs. Eventually, the self-assembly behaviors and the microstructures in the obtained solid-state PDMS-µ-PCL µ-SCPs were investigated by small-angle X-ray scattering (SAXS).

Characterization
The progress of monomer conversion was tracked using a Hewlett Packard 5890 series II gas chromatograph (GC) featuring an FID detector and a CNW CD-5 column (30 m), with toluene employed as an internal standard. Gel permeation chromatography (GPC) was carried out at 40 • C in THF (flow rate: 1 mL/min) using a Waters 515 pump, a Waters 410 differential refractometer, a Waters 486 absorbance detector, and two PSS SDV columns (Linear S and 100 Å pore size) to determine M n , M w and M w /M n (i.e., PDI). Monodisperse polystyrene (PSt) standards were used for calibrations. Recycling preparative gel permeation chromatography (rGPC) was carried out at ambient temperature in THF (flow rate: 10 mL/min) using a LaboACE LC-5060 equipped with a P-LA60 pump, a RI-700 LA differential refractometer, and JAIGEL-2.5HR columns (exclusion limit 20,000). Polymer solutions (10 mg/mL) were purified to obtain low PDI samples. FT-IR spectra were captured using a Nicolet Avatar 320 FT-IR spectrometer operating at a resolution of 4 cm −1 , with 24 scans. The samples were dissolved in THF and applied onto a KBr plate. Proton nuclear magnetic resonance ( 1 H NMR) spectra were acquired using a Varian 400 NMR and calibrated against an internal standard of CDCl 3 (δ = 7.26 ppm). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was recorded on a Bruker Autoflex III spectrometer plus in the reflectron ion mode. Trans-2-[3-(4tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) and sodium chloride (NaCl) were, respectively, used as the matrix and ionizing agent for the MALDI-TOF mass measurements. The dried samples were placed in an N 2 -purged chamber to maintain environmental dryness. Small-angle X-ray scattering (SAXS) analysis was conducted at the Endstation BL23A1 National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The X-ray source operated at 15 kV, and the sample-to-detector distance was 3 m. The scattering profile, obtained using a Pilatus-1MF detector, was plotted in terms of scattering intensity (I) as a function of the scattering vector magnitude [q = (4π/λ) sin (θ/2)]. The d-spacing was determined from the first-order scattering peak (q*) using the formula d = 2π/q*.

Synthesis of PDMS-VBA m -PDMS through Styrenics-Assisted Atom Transfer Radical Coupling (SA ATRC)
In an example of the SA ATRC reaction, CuBr (0.43 g, 3 mmol), toluene (100 mL), and a stirring bar wrapped with fresh copper wire (5 cm) were added to a septum-sealed Schlenk flask. The mixture was subsequently deoxygenated by nitrogen purging treatment for 60 min. A mixture of VBA (3.8 mL, 30 mmol), PDMS-Br (3.8 g, 3 mmol with M n,0 = 1280, PDI = 1.14), and Me 6 TREN (2 mL, 7.5 mmol) was then injected into the flask after treating in the nitrogen atmosphere and the reaction was cooled to 0 • C. Samples were taken periodically under a nitrogen blanket and passed through a short column of neutral alumina to remove dissolved copper salts before the analysis of GPC. The reaction was exposed to an ambient atmosphere and the crude product was diluted with THF and passed through a neutral alumina column. The solution was concentrated under a vacuum to remove the residual solvent, monomer, and ligand. The collected polymer was vacuum-dried to afford PDMS-VBA m -PDMS (M n = 2440, PDI = 1.31, yield 25%; coupling efficiency (x c ) = 0.95 where x c = 2 × (1 − M n,0 /M n )).

Chain Extension of PDMS-VBA m -PDMS with ε-CL through ROP
An oven-dried Schlenk flask and magnetic stirrer bar with DPP (50 mg, 0.2 mmol) and PDMS-VBA m -PDMS (0.5 g, 0.2 mmol) and toluene (10 mL) were charged. The flask was sealed and deoxygenated using freeze-pump-thaw for three cycles and refilled with nitrogen. ε-CL (1.1 mL, 9.8 mmol) was injected into the solution ([ε-CL] 0 = 0.8 M), and the polymerization was allowed to commence at 40 • C. An initial sample was taken via a syringe to trace the chain extension through ROP at ambient temperature. The reaction was stopped by adding an acidic Amberlyst ® A21 and diluted with THF. The ion-exchange resin was removed by filtration and the clear solution was precipitated in MeOH. Using similar procedures, two crude PDMS-µ-PCL were collected and dried under vacuum (i.e., M n,GPC = 4300, PDI = 1.72, yield 75%; M n,GPC = 7990, PDI = 1.52, yield 73%). These samples were further purified by rGPC to remove the impurities and well-defined PDMSµ-PCL samples can be acquired (i.e., µ-SCP1: M n,GPC = 7140, PDI = 1.24 and µ-SCP2: M n,GPC = 11,230, PDI = 1.20). Figure 1A demonstrates the GPC traces of PDMS-OH and PDMS-Br. Before conducting effective SA ATRC [49][50][51][52], the PDMS-Br precursor was synthesized through the acylation of the PDMS-OH with 2-bromoisobutyryl bromide (Scheme S1c (see the ESI)) [47]. The GPC traces of the corresponding PDMS-OH and PDMS-Br products (i.e., curves a1 and a2 in Figure 1A) showed only a minor shift and remained similar in molecular weight (MW) characteristics. After acylation of PDMS-OH, the MW slightly shifted from 1250 to 1280 with low PDI values (PDI < 1.17). Characterized by IR spectroscopy, Figure S2a (see the ESI) displays the bending of the Si-CH 3 bond, Si-O, Si-CH 3 bond, and the stretching of the C-H bond (790, 1010, 1260, and 2960 cm −1 , respectively). As shown in Figure S2b (see the ESI), the stretching of C=O (1740 cm −1 ) was additionally observed, revealing the successful acylation of the PDMS-OH. As shown in Figure 1B VBA/PDMS-Br were then conducted to examine the efficacy of SA ATRCs (VBA/PDMS-Br/CuBr/Me6TREN = x/1/1/2.5 at 0 °C with 0.05 cm Cu wire/mL toluene (x = 0, 1, 4, and 10)). Without VBA ( Figure 1B), the Mn increased to approximately 1830 with a low PDI value (1.33) after SA ATRC of PDMS-Br. The coupling efficiency (xc) can be estimated to be ca. 0.6 (where xc = 2 × (1 − Mn,0/Mn), Mn,0: the initial number average MW, Mn: the number average MW after coupling). For SA ATRCs with excess amounts of VBA ( Figure 1C,D), the Mns increased to approximately double the Mn,0 (ca. 2450) with low PDI values (<1.30).

Results and Discussion
High xc values of approximately 0.95 were acquired in both cases. With a high ratio of VBA/PDMS-Br = 10, the result stayed at the coupling reaction instead of the chain extension of the PDMS-Br with VBA. This is ascribed to the low-temperature condition for SA ATRC.
To further understand the structural details after SA ATRC, the PDMS-Br precursor and a well-coupled PDMS-VBA m -PDMS polymer were analyzed by MALDI-TOF MS using DCTB as the matrix and NaCl as the ionizing agent. Figure 3a displays the MALDI-TOF spectrum of PDMS-Br (M n = 1280, PDI = 1.14). Interestingly, a series of major peaks (i.e., star marks) with a range of approximately 222 Da, corresponding to the hexamethyl trisiloxane (HMTS) molar mass, was observed. This should intrinsically originate from the ROP of the 6-member ring of HMTS for PDMS preparations. Taking an example of the peak at m/z = 1131.5, the PDMS-Br with rational functionalities can be deduced (i.e., m/z (cal. Figure 2 displays 1 H NMR spectroscopy analysis of the PDMS-based polymers and their assigned relevant characteristic peaks. Compared the spectra of 2A and 2B, quantitative chain end modification from PDMS-OH to PDMS-Br can be confirmed (i.e., disappearance of peaks a (~3.65 ppm (t, -CH2CH2OH, 2H)) and b (~3.45 ppm (t, -CH2CH2OH, 2H)) and appearance of peaks a' (~4.32 ppm (t, -CH2OCO-, 2H)), b' (~3.67 ppm (t, -CH2CH2OCO-, 2H)), and j (~1.94 ppm (s, -OCOC(CH3)2Br, 6H))). After SA ATRC of PDMS-Br with VBA ( Figure 2C), the disappearance of peak j and appearance of peaks k and l were observed (~4.6 and 6.8-7.2 ppm, respectively). The 1 H NMR spectra implied the occurrence of the convergent reaction that inserted only a few VBA units. To further understand the structural details after SA ATRC, the PDMS-Br precursor and a well-coupled PDMS-VBAm-PDMS polymer were analyzed by MALDI-TOF MS using DCTB as the matrix and NaCl as the ionizing agent. Figure 3a displays the MALDI-TOF spectrum of PDMS-Br (Mn = 1280, PDI = 1.14). Interestingly, a series of major peaks (i.e., star marks) with a range of approximately 222 Da, corresponding to the hexamethyl trisiloxane (HMTS) molar mass, was observed. This should intrinsically originate from the ROP of the 6-member ring of HMTS for PDMS preparations. Taking an example of the peak at m/z = 1131.5, the PDMS-Br with rational functionalities can be deduced (i.e., m/z The scenario can be rationalized based on kinetic details to discuss such interesting results. For simplicity, the rate constants of activation (k act ), deactivation (k dea ), and termination (k t ) of PDMS-Br with the bromoisobutyryl group are classified to those of the similar structures of ethyl 2-bromoisobutyrate (EBiB) and methyl methacrylate (MMA) (i.e., k act,EBiB , k dea,EBiB , and k t,MMA , respectively). For the role of VBA, the rate constants were amended from 1-phenylethylbromide (PEBr) and styrene (St) (i.e., k act,PEBr , k dea,PEBr , k p , and k t,St ). Table 1 summarizes the relevant reaction rate constants. In the absence of VBA, disproportionation termination between PDMS• methacrylic macroradicals was dominated [53,54], leading to the formation of PDMS-= and PDMS-H products. Once VBA was added, disproportionation termination of the PDMS• methacrylic macroradical was significantly suppressed and guided the overall reaction to conduct radical-radical coupling based on the following kinetic balances: (i) A high k dea,EBiB (= ca. 5.1 × 10 5 M −1 s −1 ) [55][56][57] that can conduct efficient deactivation reaction and thus keep the PDMS-Br chain-end integrity; (ii) A rapid cross-reaction between PDMS• and VBA (k cr = ca. 2.5 × 10 3 M −1 s −1 ) [58] to significantly transform the nature of the methacrylic macroradical to styrenic macroradical; (iii) The propagation rate constant of VBA is maintained at a significantly low value of approximately 40 M −1 s −1 at 0 • C. This critical control guarantees minimal chain extension during the reaction, thereby minimizing the consecutive addition of VBA to the newly-formed PDMS-VBA• macroradical (i.e., resulting in nearly a monoadduct). The prompt coupling reaction can be effectively dominated (k t,St = ca. 10 8 M −1 s −1 ) due to the nature of the styrenic radical termination [54,59,60]. Rationally, the k act,PEBr and k dea,PEBr are relatively low, which would be insufficient to establish the activation-deactivation reactions of PDMS-VBA-Br. Thus, PDMS-VBA 2 -PDMS was mainly formed.  The scenario can be rationalized based on kinetic details to discuss such interesting results. For simplicity, the rate constants of activation (kact), deactivation (kdea), and termination (kt) of PDMS-Br with the bromoisobutyryl group are classified to those of the similar structures of ethyl 2-bromoisobutyrate (EBiB) and methyl methacrylate (MMA) (i.e., kact,EBiB, kdea,EBiB, and kt,MMA, respectively). For the role of VBA, the rate constants were amended from 1-phenylethylbromide (PEBr) and styrene (St) (i.e., kact,PEBr, kdea,PEBr, kp, and kt,St). Table 1 summarizes the relevant reaction rate constants. In the absence of VBA, disproportionation termination between PDMS• methacrylic macroradicals was dominated [53,54], leading to the formation of PDMS-= and PDMS-H products. Once VBA was added, disproportionation termination of the PDMS• methacrylic macroradical was significantly suppressed and guided the overall reaction to conduct radical-radical coupling based on the following kinetic balances: (i) A high kdea,EBiB (= ca. 5.   After achieving high efficiency of SA ATRC with VBA as a coupling agent, mid-chain functionalized polymers with benzylic alcohols were obtained. The PDMS-VBA 2 -PDMS MI with two hydroxyl groups allows us to perform chain extension with ε-CL through living-ROP (ε-CL/PDMS-VBA 2 -PDMS/DPP = 50/1/1 at 40 • C; [ε-CL] 0 = 0.8 M). As shown in Figure 4A, the kinetics revealed a linear first-order plot with an apparent rate constant (k app ) of approximately 1.03 × 10 −4 s −1 . Figure 4B showed moderate increases in PDIs and Figure 4C showed gradual increases in MWs and the M n s relating to conversion. As shown in Figure 4D, the GPC traces showed significant growth in MW. However, a shoulder was observed after 2 h, which might be attributed to the unignorable self-polymerization of ε-CL via the cationic catalyst. To obtain the well-defined PDMS-µ-PCL µ-SCPs with low PDI, we employed a recycling GPC (rGPC) to separate the undesired portions. As shown in Figure S3    The obtained PDMS-µ-PCL µ-SCP was also subjected to structural elucidation using 1 H NMR spectroscopy. Figure 6A displays the spectrum of PDMS-VBA 2 -PDMS MI, which contains benzylic alcohol (-Ph-CH 2 -OH) at peak b (4.4-4.7 ppm). In Figure 6B, the spectrum of PDMS-µ-PCL µ-SCP reveals the presence of characteristic signals from the PDMS backbone. Additionally, peaks b', j, l, and q from benzyl ester and PCL backbone (4.9-5.2, 4.06, 2.30, 1.2-1.7 ppm, respectively) were observed, while peak b corresponding to the benzylic alcohol disappeared. Two samples were thus acquired and the volume fractions of PDMS in µ-SCP1 and µ-SCP2 were estimated as f v PDMS = 0.46 and 0.32, respectively. The successful acquisition of µ-SCP1 (M n,GPC = 7140, PDI = 1.24) and µ-SCP2 (M n,GPC = 11,230, PDI = 1.20) was achieved using the rGPC instrument, and both samples exhibited well-defined structures (see again curves b and d in Figure 5).  The obtained PDMS-μ-PCL μ-SCP was also subjected to structural elucidation using 1 H NMR spectroscopy. Figure 6A displays the spectrum of PDMS-VBA2-PDMS MI, which contains benzylic alcohol (-Ph-CH2-OH) at peak b (4.4-4.7 ppm). In Figure 6B, the spectrum of PDMS-μ-PCL μ-SCP reveals the presence of characteristic signals from the PDMS backbone. Additionally, peaks b', j, l, and q from benzyl ester and PCL backbone (4.9-5.2,   Figure 5). The obtained μ-SCP1 and μ-SCP2 were further investigated for their solid-state selfassembly. Bulk film samples were individually prepared by solution-casting method (5 wt% μ-SCPs/THF solution) on Teflon plates. The specimens were dried under ambient temperature for 24 h and then thermal annealed at 60 °C under vacuum for 12 h. The obtained bulk film samples were measured using SAXS under ambient conditions to analyze their microstructure. As shown in Figure 7A, a lamellar (LAM) structure (i.e., q/q* peaks of 1:2) were obtained in the annealed sample of μ-SCP1 [(PDMS13)2-μ-(PCL20)2, f v PDMS = 0.46]. The d-spacing of the nanostructures was approximately 15.3 nm. As shown in Figure 7B, a hexagonally packed cylinder (HEX) structure (i.e., q/q* peaks of 1:√3) having a periodic size of approximately 16.5 nm was detected in the case of the μ-SCP2 sample The obtained µ-SCP1 and µ-SCP2 were further investigated for their solid-state selfassembly. Bulk film samples were individually prepared by solution-casting method (5 wt% µ-SCPs/THF solution) on Teflon plates. The specimens were dried under ambient temperature for 24 h and then thermal annealed at 60 • C under vacuum for 12 h. The obtained bulk film samples were measured using SAXS under ambient conditions to analyze their microstructure. As shown in Figure 7A, a lamellar (LAM) structure (i.e., q/q* peaks of 1:2) were obtained in the annealed sample of µ-SCP1 [(PDMS 13 ) 2 -µ-(PCL 20 ) 2 , f v PDMS = 0.46]. The d-spacing of the nanostructures was approximately 15.3 nm. As shown in Figure 7B, a hexagonally packed cylinder (HEX) structure (i.e., q/q* peaks of 1: √ 3) having a periodic size of approximately 16.5 nm was detected in the case of the µ-SCP2 sample [(PDMS 13 ) 2 -µ-(PCL 38 ) 2 , f v PDMS = 0.32]. It is worth noting that the "bottom-up" approaches to forming significantly ordered nanostructures by block copolymer (BCP) have a common strategy to use overall high molecular weight copolymers (i.e., high degrees of polymerization (N)) to conduct self-assembly behaviors. It is generally because most polymer-polymer interaction parameters (χ) are small. However, Kim et al. [62]. introduced an alternative strategy using a star block copolymer (SBCP) with 18-armed PMMA-b-PSt (i.e., (PMMA-b-PSt) 18 ). They introduced a self-neutralization approach to achieve structured nanoarchitectures by increasing the "entropy penalty". By innovatively controlling the polymer architecture in such a low χ system (ca. 0.04) [63], they beautifully fabricated oriented lamellar and cylindrical nanostructures ranging from approximately 50-150 nm on silicon wafers. Similarly, Ho et al. [64] combined two strategies to produce aligned structures (i.e., low entropy and large χ). By controlling the polymer architecture of (PSt x -b-PDMS y ) n SBCPs (arm numbers (n) = 1-4 and repeating units (x + y) < 20), they ensured a high value of χ (ca. 0.27) between the blocks. As a result, these SBCPs spontaneously aligned into the cylinder and lamellar structures of sub-20 nm feature size on silicon wafers. A similar two-strategies method was interestingly demonstrated by a unique system of maltoheptaose-based SBCPs [65,66], which showed sub-10 nm feature size. Our PDMS-µ-PCL system has the attributes of low N, large χ, and low entropy. Accordingly, such peculiarities can easily lead to performing self-assembly on a sub-nano scale. By minimizing the molecular spatial stability through MM2 molecular modeling method, contour lengths (L) of the µ-SCPs can be estimated. As shown in Figure S4 (see the ESI), L µ-SCP1 = 22.7 and L µ-SCP2 = 36.9 nm were acquired based on a set of end groups, PDMS n segments, PCL p segments, linkages, and VBA units. Figure 8 displays our proposed microstructure to indicate the roles and features of PDMS (i.e., only in the form of amorphous) and PCL (i.e., with the forms of amorphous and crystalline) chains. Consequently, the measured periodic dimensions of the µ-SCPs using SAXS are smaller than the estimated contour lengths. This phenomenon can be logically attributed to the strong crystallization integrity of the PCL chains, leading to a densely packed PCL-rich area, whether in a LAM or HEX structure. Our ongoing efforts involve examining the intrinsic properties and self-assembly tendencies of these µ-SCPs. More comprehensive insights and specific findings will be unveiled in an upcoming publication.
Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 16 achieve structured nanoarchitectures by increasing the "entropy penalty". By innovatively controlling the polymer architecture in such a low χ system (ca. 0.04) [63], they beautifully fabricated oriented lamellar and cylindrical nanostructures ranging from approximately 50-150 nm on silicon wafers. Similarly, Ho et al. [64] combined two strategies to produce aligned structures (i.e., low entropy and large χ). By controlling the polymer architecture of (PStx-b-PDMSy)n SBCPs (arm numbers (n) = 1-4 and repeating units (x + y) < 20), they ensured a high value of χ (ca. 0.27) between the blocks. As a result, these SBCPs spontaneously aligned into the cylinder and lamellar structures of sub-20 nm feature size on silicon wafers. A similar two-strategies method was interestingly demonstrated by a unique system of maltoheptaose-based SBCPs [65,66], which showed sub-10 nm feature size. Our PDMS-μ-PCL system has the attributes of low N, large χ, and low entropy. Accordingly, such peculiarities can easily lead to performing self-assembly on a sub-nano scale. By minimizing the molecular spatial stability through MM2 molecular modeling method, contour lengths (L) of the μ-SCPs can be estimated. As shown in Figure S4 (see the ESI), Lμ-SCP1 = 22.7 and Lμ-SCP2 = 36.9 nm were acquired based on a set of end groups, PDMSn segments, PCLp segments, linkages, and VBA units. Figure 8 displays our proposed microstructure to indicate the roles and features of PDMS (i.e., only in the form of amorphous) and PCL (i.e., with the forms of amorphous and crystalline) chains. Consequently, the measured periodic dimensions of the μ-SCPs using SAXS are smaller than the estimated contour lengths. This phenomenon can be logically attributed to the strong crystallization integrity of the PCL chains, leading to a densely packed PCL-rich area, whether in a LAM or HEX structure. Our ongoing efforts involve examining the intrinsic properties and self-assembly tendencies of these μ-SCPs. More comprehensive insights and specific findings will be unveiled in an upcoming publication.

Conclusions
We employed a styrenic coupling agent, VBA, in the SA ATRC process to obtain PDMS-VBAm-PDMS products. By analyzing the outcomes through GPC, we achieved a high coupling efficiency (χc = 0.95) with a monomodal MW distribution. Intriguingly, MALDI-TOF MS analysis of the PDMS-VBAm-PDMS product confirmed the insertion of only two VBA units (m = 2). To extend chains, we subjected PDMS-VBA2-PDMS to midchain extensions with ε-CL through ROP, employing the organo-catalyst DPP. To remove any undesirable side products, such as linear PCL homopolymer, we employed a rGPC process. This allowed us to acquire well-defined μ-SCPs with low PDIs (<1.30). Further examination of the obtained PDMS-μ-PCLs through SAXS measurements revealed microphase separations, with LAM and HEX structures observed in bulk film samples of μ-SCP1 (Mn,GPC = 7140, f v PDMS = 0.46) and μ-SCP2 (Mn,GPC = 11,230, f v PDMS = 0.32), respectively. The achievement of sub-15 nm microstructures can be attributed to the attributes of low PDI (<1.3), low N of arms (PDMS13 and PCL20/PCL38), large χ, and low entropy in the μ-SCPs. In brief, our approach demonstrates high coupling efficiency and provides a means to introduce mid-chain functionality for conducting post-reactions. This convergent strategy can be extended to synthesize architecturally distinct and well-defined novel macromolecules. The characterization of structural details and diversities is currently underway. We will present the relevant data shortly.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Scheme S1. Reaction steps for the syntheses of (a

Conclusions
We employed a styrenic coupling agent, VBA, in the SA ATRC process to obtain PDMS-VBA m -PDMS products. By analyzing the outcomes through GPC, we achieved a high coupling efficiency (χ c = 0.95) with a monomodal MW distribution. Intriguingly, MALDI-TOF MS analysis of the PDMS-VBA m -PDMS product confirmed the insertion of only two VBA units (m = 2). To extend chains, we subjected PDMS-VBA 2 -PDMS to mid-chain extensions with ε-CL through ROP, employing the organo-catalyst DPP. To remove any undesirable side products, such as linear PCL homopolymer, we employed a rGPC process. This allowed us to acquire well-defined µ-SCPs with low PDIs (<1.30).  13 and PCL 20 /PCL 38 ), large χ, and low entropy in the µ-SCPs. In brief, our approach demonstrates high coupling efficiency and provides a means to introduce mid-chain functionality for conducting post-reactions. This convergent strategy can be extended to synthesize architecturally distinct and well-defined novel macromolecules. The characterization of structural details and diversities is currently underway. We will present the relevant data shortly.